acid rain

Most rainfall is generally slightly acidic due to the carbonic acid from carbon dioxide in atmosphere. But 'acid rain' is caused when sulfur (sulphur) dioxide and nitrogen oxides (from automobile exhausts and industrial emissions) are washed out from the atmosphere by rain as weak sulfuric (sulphuric) and nitric acid. Acid rain can cause serious damage to crops, and leaches calcium ions from soil and plant leaves causing an ionic imbalance. See also acid deposition.